1. Field of the Invention
The present invention relates to an image reading apparatus for a digital copying machine, a facsimile machine, a scanner, or the like, and an image reading method executed by the image reading apparatus, and more particularly to an image reading apparatus configured to be capable of performing simultaneous double-sided reading for simultaneously reading both sides of an original during one-time conveyance of the original, and an image reading method executed by the image reading apparatus.
2. Description of the Related Art
Recently, a series of system developments concerning filing applications for various official certificates and notifications to public offices have been being vigorously promoted based on the electronic government concept. Such movement can also be widely seen among private companies, and with a rise in interest in security of personal information and an increase in demand for security of secret information, attention has been just increasingly given to security techniques for paper documents.
A typical example of the security techniques is a forgery prevention technique, and also mentioned as important security techniques are a security printing technique of disabling copying of a secret document by embedding a fine and complicated structure in image data and a determination technique of accurately reading a security document to thereby determine whether or not the security document is a secret document.
In offices, copying machines are most generally used for printing paper documents, and means for preventing forgery of paper money and unauthorized copying of secret documents are indispensable to the copying machines. Especially, the image reading apparatus of each copying machine is required to perform highly accurate reading.
As an image reading apparatus used e.g. in a copying machine, there has been known one which performs so-called “moving original reading” in which originals are conveyed onto an original platen glass, one by one, by an automatic document feeder and an original image on each of the originals is read by exposing the original to light by an exposure device fixedly disposed on the conveying path. This technique has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2001-285595. Further, another image reading apparatus has been known in which two image readers are provided for enhancement of the productivity of the image reading apparatus, so as to enable both sides of an original to be read during one-time conveyance of the original (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2004-187144).
However, in the image reading apparatus configured to read both sides of an original during one-time conveyance, differences between image reading positions of the respective image readers can cause variation in the amount of light used for double-sided original reading. Further, differences (front-side and reverse-side reading level differences) can occur between read values obtained by reading the front side of an original and read values obtained by reading the reverse side of the original.
To correct the front-side and reverse-side reading level differences and the reading position-dependent variation in the light amount in the double-sided original reading, there has been proposed a method in which shading correction is performed using reference white plates so as to obtain equal values even in different reading modes. This technique is disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2001-157052.
However, to perform matching of reading characteristics for correction of shading correction data, Japanese Laid-Open Patent Publication (Kokai) No. 2001-157052 specifically discloses nothing more than a technique of performing uniform gain adjustment in the main scanning direction.
To read such a paper document formed by the additional use of a security technique as described above, strict in-surface uniformity (uniformity over one surface of an original) of the read data is required, and hence the correction technique mentioned above is not enough to cope with the reading of such a security document. Further, the correction technique also suffers from the problem of being incapable of coping with a case in which the front side of an original and the reverse side of the same are largely different in reading characteristics.
Furthermore, an image reading apparatus employing the correction technique is required to have not only a system for shading correction, but also a circuit for correcting shading correction data, which inevitably makes the apparatus large-sized and complicated in configuration, thereby causing an increase in manufacturing costs. Particularly in the case of reading both sides of an original by the moving original reading method, read values obtained by reading the front side of an original differ from read values obtained by reading the reverse side of the same, which brings about the problems of a density difference occurring in monochrome original copying and a color difference occurring in color original copying.